Ca2+-binding properties and regulatory roles of lobster troponin C sites II and IV

Invertebrate troponin C typically contains Ca2+-specific binding sites, sites II and IV, in the N- and C-terminal domains, respectively. To investigate the roles of these sites for Ca2+-dependent regulation of muscle contraction, we generated lobster troponin C mutants, and analyzed their Ca2+-binding properties and regulatory effects on actomyosin-tropomyosin Mg-ATPase activity. The results suggest that Ca2+ binding to site IV is responsible for regulation at relatively low Ca2+ concentrations, while site II has an essential role in full activation at higher Ca2+ concentrations.